1. Field of the Invention
The present invention relates to a method for fabricating ion exchange waveguides in optical modulators using pressurized annealing and the resulting waveguides and modulators. More particularly, the present invention relates to a method for fabricating lithium niobate-ion or lithium tantalate-ion exchange waveguides using a pressurized oxygen atmosphere anneal process to further diffuse ions in the exchange region.
2. Background
Optoelectronic components can be fabricated on several types of substrates including polymers, glass, semiconductors (e.g., gallium arsenide (GaAs) and indium phosphide (InP)) and inorganic materials (e.g., lithium niobate (LiNbO3) and lithium tantalate (LiTaO3)). Characteristically, an electro-optic material is one in which the index of refraction changes with the application of an electric field. One of the most important components in optoelectronic systems is the modulator. Three competing technologies in this realm are: direct modulation of a semiconductor laser, semiconductor electro-absorption modulators, and the lithium niobate modulator. Currently, lithium niobate modulators are the modulation devices of choice for many systems because they yield high performance, are a relatively mature technology and other modulation schemes impose limits not faced with lithium niobate modulators.
Lithium niobate has proven to be a suitable medium for components such as amplitude modulators, phase modulators, optical switches, polarization transformers, tunable filters and wavelength-selective optical add/drop filters. Lithium niobate has also been used as the host for solid state lasers using rare earth ions, e.g., erbium. Most current telecommunication and cable television system applications for LiNbO3 modulators involve discrete components for the optical transmitter subsystem. This configuration couples continuous wave lasers, typically diode-pumped YAG or erbium fiber oscillators, with lithium niobate external modulators and various wavelength and power stabilization components.
Lithium niobate is a popular nonlinear optical crystal for several reasons including its large electro-optic coefficients, the ease with which high quality optical waveguides are fabricated and its amenability to conventional integrated circuit processing techniques. High quality optical waveguides are those that possess low loss and relatively high power handling capabilities. Additionally, LiNbO3 is a hard material, thus it is easily polished for fiber optical coupling which makes its use in optical network systems relatively uncomplicated. It is also a relatively inexpensive crystal, due in part to its long history of use in surface-acoustic-wave (SAW) filters for radio frequencies. By comparison, lithium tantalate LiTaO3 is essentially interchangeable with lithium niobate as far as modulator characteristics are concerned, but the use of LiTaO3 is often cost prohibitive because it is not as widely commercially used as LiNbO3. Additionally, other optical crystalline structures having the formula RMO3, where R is an alkaline earth metal, M is a Group IVB or Group VB metal, and O is oxygen, can also be used in the fabrication of waveguides and modulators.
For example, one type of modulator incorporating the use of LiNbO3 is a Mach-Zehnder modulator. In a Mach-Zehnder modulator an incoming optical beam is split equally at a Y junction into two parallel waveguides, which then recombine at another Y junction after some distance. Electrodes are used to apply an electric field in the region of the optical waveguides. The electric field causes a relative phase shift between the two branches. If the phase shift in both arms is the same, then the two optical signals will recombine constructively at the second Y branch and no optical power will be lost. If there is a phase difference between the two arms, then there will be destructive interference and some optical power will be lost. The resulting destructive and constructive interference causes the output optical intensity to vary between a minimum and a maximum value.
In other electro-optic applications optical waveguide directional couplers can also be used as electro-optic modulators. In this type of modulator two waveguides are placed on the lithium niobate substrate in very close proximity to one another. The wave from one guide can “leak” into the other guide. By applying an electric field to the lithium niobate, the amount of “leakage” can be controlled, thus modulating the transfer of power from one guide to the other. Currently, different commercial application requirements favor either directional couplers or Mach-Zehnder modulators.
The advance of high-speed, large bandwidth, digital and analog communications has led to a demand for the external modulator configuration. The most common approach utilizes a low-noise, high power diode-pumped laser whose signal is sent to the LiNbO3 modulator via optical fiber. The modulator applies either analog or digital information to the optical carrier.
When using lithium niobate in the fabrication of optical waveguides and optical modulators it is desired to avoid having a niobium-rich, lithium-poor and/or oxygen poor composition. When bulk lithium niobate has such niobium rich-compositions, and is then processed at high temperatures (T>300 degrees Celsius), growth of the LiNb3O8 phase in the crystal may occur. This phase is undesirable because it is not optically transparent and leads to high losses in optical waveguides and optical modulators.
Such niobium-rich compositions can occur in two different manners during fabrication of optical waveguides and optical modulators. First, typical ion exchange procedures result in the replacement of lithium atoms in the crystal lattice with a diffusing ion leading to a lithium niobate composition relatively rich in niobium. Second, standard high temperature (temperatures in excess of 300 degrees Celsius) processing of lithium niobate can cause Li2O out-diffusion, and result in niobium-rich, and lithium and oxygen poor compositions.
To eliminate the undesired LiNb3O8 phase from forming in the crystal, high temperature processing, such as the stress relieving anneal process, is usually performed in a wet atmosphere of inert carrier gas, such as nitrogen (N2) or argon (Ar2), or in a wet atmosphere of oxygen (O2). This type of anneal process involves bubbling the inert carrier gas or oxygen gas through water (H2O). The wet atmosphere has been considered beneficial in the past because the H2O breaks down into H+ and OH− ions which chemically attack the LiNb3O8 phase and break it back down into LiNbO3. A typical wet atmosphere anneal operation is performed at a temperature of about 350 degrees Celsius for a period of 5 to 6 hours in a wet, flowing environment. The present inventor has discovered that a drawback of this type of high temperature processing is that the H2O gives off undesirable protons (H+) which are attracted by the lithium niobate and result in an inadvertent proton-exchanged surface layer occurring. These protons remain in the modulators after fabrication and flow relatively freely among the waveguides, the electrodes, the lithium niobate crystal and the buffer layer interface. It is now understood that these free flowing protons can and do adversely affect a modulator's DC-bias stability because they act as charge carriers and are driven by the applied electric fields, causing the response of the final product to drift over time when a bias is applied.
Additionally, high temperature processing (>300 degrees Celsius) leads to oxygen out-diffusion of lithium niobate structures. This out-diffusion tends to form a surface layer on a lithium niobate substrate or a lithium niobate waveguide that is oxygen poor in composition. This oxygen poor region acts as a surface optical waveguide and causes undesirable leakage of light out of the lithium niobate substrate.
Ion exchange waveguides have typically been fabricated by treating or exchanging the surface of the crystalline substrate with a source of ions. In most applications the exchanging has been performed with protons in the form of an acid, such as sulfuric acid or benzoic acid. The exchanging allows for the initial diffusion to take place, resulting in up to about 50% ion exchange (i.e., the ions replace the alkaline earth metal atoms in the crystal lattice). The initial exchanging process is then followed by a procedure that will further diffuse the ions and drive the diffusion region farther into the depth of the crystalline substrate. Ion exchange can also be accomplished by using alkaline earth metal salt (i.e., lithium salt) as a buffer to the exchanging acid. Salt treatments are inefficient because they are time prohibitive, some salt treatments can take upwards of 48 hours. Standard wet anneal processes introduce undesirable protons which are attracted by the crystalline substrate and result in an inadvertent proton exchanged surface layer occurring.
It would therefore be highly advantageous to devise a fabrication method for ion exchange waveguides that uses a new high temperature annealing process that inhibits both the formation of the undesirable LiNb3O8 phase in the crystal and outdiffusion of O2 without the process introducing significant numbers of free flowing protons that will affect the modulator's DC-bias stability.